Control system for monitoring and adjusting combustion performance in a cordwood-fired heating appliance

ABSTRACT

A system including temperature measuring devices, a controller, and an actuator, maintains a desired gas temperature range in the secondary combustor by proportionally making adjustments to the primary air orifice or other primary air valve of a cordwood burning appliance. Heat output of the appliance is directly related to the temperature of the gases in the secondary combustor, and sufficient engagement of the second combustor during off-gassing of volatiles from the wood will control emissions from the appliance. The operator can select a desired room temperature via the thermostat and the controller will use different secondary gas temperature target ranges in order to meet the desired room temperature as commanded by the thermostat throughout the burn event. The controller also controls the start-up phase of combustion and charcoal phase of combustion.

DESCRIPTION

This application claims priority of Provisional Application Ser. No. 61/851,343, filed Mar. 7, 2013, entitled “Control System for Monitoring and Adjusting Combustion Performance in a Cord Wood Fired Residential Heating Appliance”, the entire disclosure of which is incorporated herein by this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to control systems designed to operate on wood stoves, furnaces, fire places, and/or other types of appliances that are designed to burn solid fuel, preferably wood, and that are equipped with secondary combustion systems, frequently called “secondary combustors”. Certain embodiments are for residential wood-burning appliances. Certain embodiments are designed to optimize burn characteristics to reduce emissions, maximize efficiency, and make operation simple for the operator.

2. Discussion of Related Art

Cordwood burning appliances must contend with a huge amount of variation in physical and environmental factors that is realized in the field. These physical and environmental variables may include, for example, different operators, wood type, wood moisture content, load density, altitude, chimney installation, etc., and may all have impacts on the particulate emissions profile that the appliance will produce while in operation. Typically, modern wood-burning appliances have already been “tuned” to an EPA mandate and tested in a laboratory for particulate emissions from the appliance. However, this “standard” does not reflect real world conditions, and likely never will, as the number of permutations needed to understand the field emissions profile, based on the aforementioned contributing factors, are far too many. There is increasing pressure from air regulatory authorities to clean up air sheds, but, given that laboratory testing and realized field emissions do not and will likely never correlate, the only tool regulators currently have is to lower the passing threshold for new appliances coming to market.

Wood combustion may be categorized generally into three phases of combustion: the start up or “wood alcohol” phase, the organic release phase or “organic phase”, and, finally, the charcoal phase. Wood combustion, through all phases, is a dynamic phenomenon, and each refueling event into an appliance is a different event. The inventor believes that the only things consistent from wood-burning event to event are that wood will burn and volatiles (combustible gaseous compounds from the wood) may burn in the secondary combustor under some circumstances. Therefore, success in obtaining a low emissions profile during operation of a wood burning appliance can be difficult for operators. Units generally will not operate with minimal emissions unless a skilled operator remains present to make adjustments. Due to the manual system that most appliances employ, and the many physical and environmental variables mentioned above, the operator must stay with the appliance, “watching” its performance during the various stages of the combustion process, and making adjustments to the air inlet control. For example, if the air control for the appliance is closed off prematurely in order to provide a low heat output and prolong the length of the burn, then excessive emissions and low efficiency are often the result. Therefore, the multiple goals, of trying to heat a room comfortably during use of the room, burning wood cleanly, extending length of a burn, and/or use less wood, are entirely at the discretion of the operator and are seldom all achievable by conventional, manual equipment and methods.

Therefore, there is a need for improved control of solid-fuel-/wood-burning appliance operation, to improve multiple, and preferably all, of emissions, comfort, and convenience. Embodiments of the invention meet this need, using the temperature of the secondary combustor gases (or “secondary combustion temperature”, SCT), which indicate said level of secondary combustor engagement, to control air during at least one combustion phase. With the secondary combustor engaged to a certain level, the combustion event will tend to exhibit minimal emissions and increased efficiency due to heat production from the volatiles. Certain embodiments use SCT sensing, a controller comprising control algorithm(s) specially-adapted to respond to said SCT sensing, and an actuator performing primary air adjustments in response to the controller, resulting in every refueling event exhibiting improved or optimal burn characteristics regardless of the above-mentioned physical and environmental variables. For example, improved or optimal burn characteristics may be exhibited regardless of operator, absence of operator, wood type, installation, etc.

3. Previous Attempts

Several mechanisms have been devised previously that attempt to control wood combustion in an automatic fashion. These include mechanical-type thermostats that make adjustments to the air control via referencing the temperature of the appliance cabinet, or by utilizing a timer of some sort on the primary air control. Others are electronic in nature and utilize oxygen sensors or some temperature measurement of the exhaust (flue) stream, or utilize the appliance cabinet temperature in order to make adjustments to the air control(s). None of the aforementioned systems directly measure the temperature of the gases within the secondary combustion system.

SUMMARY OF THE INVENTION

The present invention comprises apparatus, and/or methods, for automatic control of a solid-fuel-burning appliance to reduce emissions, during at least one and preferably multiple phases on combustion. Certain embodiments comprise measuring temperature of the secondary combustion system of the appliance, and controlling the amount of air into the appliance to maintain a desired level of said secondary combustion temperature(s) during at least one combustion phase. Certain embodiments comprise secondary combustion temperature (SCT) sensing apparatus, a controller responding to said SCT sensing, and an actuator adjusting the air supply to the appliance in response to the controller. Certain embodiments obtain low emissions, while also increasing efficiency and/or extending the length of the burn and providing great convenience for the operator. Certain embodiments maintain combustion in the secondary combustor that is sufficient to reduce emissions/pollution while satisfying the desired room temperature, and then, if possible while satisfying these two goals, reducing the rate of burning of wood to conserve fuel.

In certain embodiments of the invention, automatic control of air to the appliance is done chiefly or entirely by controlling the primary air flow into the appliance. In certain embodiments, this is done during all phases of combustion, with monitoring and maintaining proper gas temperatures in the secondary combustion system of the appliance being particularly important at all times wherein combustible gasses from the wood, or “volatiles”, are flowing from the primary combustion zone (also, herein, “main firebox”) to the secondary combustion zone. Time-iterative sampling of the temperature data provided from sensors in the secondary combustor allow adjustment of the primary air orifice(s)/valve(s) via an actuator. The automatic control may comprise throttling the primary air, during periods when the desired room temperature is satisfied, to an extent that reduces wood combustion to a longer-lasting burn, but that sustains secondary combustion of volatiles to reduce emissions. If room temperature is not satisfied, the automatic control may comprise increasing primary air to increase combustion of wood or charcoal in the main firebox, and of volatiles when produced, even if this results in reducing the total length of the burn.

The invented apparatus and/or methods may be retrofit, manufactured OEM, or otherwise incorporated, into various solid-fuel-burning, preferably wood-burning, appliances, such as wood fireplaces, stoves, inserts or furnaces, that include a secondary combustion system. The invented apparatus/methods are expected to work well with all current conventional, and future, secondary combustion systems, for example, those of various physical structures with or without catalytic properties. Secondary combustion systems not yet existing may use embodiments of the present invention.

Automatic control of the primary air orifice(s)/valve(s) is preferably provided during all the stages or “phases” of the combustion event, including a startup phase wherein a fresh load of solid-fuel/wood has just been added, an organic burn phase, and finally a charcoal phase. Preferably, operator input and effort is minimal other than placing wood in the main firebox and lighting the fire. In especially-preferred embodiments, the operator input to the controller comprises, consists essentially of, or consists of, selection of a desired room temperature via a wall mounted “off-the-shelf thermostat” and pressing of the “cycle start” button upon loading a fresh load of fuel.

Another inventive aspect of certain embodiments relates to what feature(s)/element(s) is/are automatic and non-adjustable to the user and what is/are adjustable. In order to provide low emissions, it is necessary to define secondary zone gas temperature presets (also “preset targets” or “targets”) that are stored in memory of the controller. Users do not have the ability to change these temperature values. In preferred embodiments, a thermostat allows a user to set a desired room temperature, and the controller/algorithm has multiple, different preset targets for secondary combustion gas temperature (SCT), the selection of which different targets is determined by whether or not the thermostat is calling for heat. A higher preset SCT target is used if room temperature is not met, so that higher heat output is achieved from the appliance to heat-up the room. A lower preset SCT target is used if room temperature is met, as one may expect that maintaining the room temperature to require a lower heat output from the appliance and that said lower preset SCT target result in a longer burn-time. The lower preset SCT target, however, is not so low that it reduces secondary combustion to an extent that causes high emissions from the secondary combustion system, and, hence, from the appliance.

These and/or other features of the invention will be apparent to those of skill in the art after reviewing this document and the attached drawings. The preferred embodiments described below are presented to show some, but not all, exemplary elements, structure, means, and methods that may be used in certain embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a wood stove with control system according to an embodiment of the present invention.

FIG. 2 is a cross-sectional side view of the wood stove with control system of FIG. 1, viewed along the line 2-2 in FIG. 1.

FIG. 3 is a bottom perspective view of the tube style secondary combustion system of the wood stove of FIGS. 1 and 2, showing a location of the temperature sensors that send data to the controller.

FIG. 4 is a front view of an alternative embodiment of wood stove with control system that comprises a catalytic-style secondary combustion system.

FIG. 5 is a side cross-section side view of the wood stove with control system of FIG. 4, viewed along the line 5-5 in FIG. 4.

FIG. 6 is a front perspective view of the catalytic unit of the wood stove and control system of FIGS. 4 and 5.

FIG. 7 is a cross-sectional side view of a wood stove such as the one in FIGS. 1 and 2 (shown without most of the control elements of embodiments of the invention), wherein the flow of primary air, gases from the burning wood, and secondary air are shown.

FIG. 8 is a graph of primary air orifice position vs time for certain embodiments capable of operating under several heat output selections, specifically selection of high, medium, or low heat output modes.

FIG. 9 is a graph of the temperature (SCT) of the gases in the secondary combustion zone/system, vs. time, generally corresponding to the low heat output mode of FIG. 8.

FIG. 10 is a graph of primary air orifice position vs time, for certain embodiments an operation wherein the SCT target starts out the burn in high heat output mode, but then switches to a low heat output mode partway through the organic combustion phase, for example, because a thermostat signals the controller that the design room temperature has been met.

FIG. 11 is a graph of the temperature (SCT) of the gases in the secondary combustion zone/system, vs. time, generally corresponding to the operation shown in FIG. 10.

FIGS. 12A, 12B, 12C are portions of a block flow diagram showing some methods of operating a wood stove comprising certain embodiments of the invented automatic control apparatus.

FIG. 13 shows additional steps that may supplement the methods of FIGS. 12A-12C, for certain embodiments wherein either: a user-operated thermostat is instrumental in causing the controller to select the SCT target range (box at top left), in that a first SCT target range is used if the thermostat is calling for heat, but a second, lower SCT target range is used if the thermostat is no longer calling for heat, or a user-operated heat output selection switch (box at top right) is instrumental in causing the controller to select the SCT target ranges.

DETAILED DESCRIPTION

Conventional appliances comprise a primary combustion chamber (also “main combustion chamber”, “main firebox”, or “firebox”) for receiving the wood or other solid fuel during the combustion event. The main firebox is an insulated volume, where the solid-fuel/wood resides, that has air passage openings that deliver primary air to one or more specific areas of the main firebox mainly for combustion of the solid fuel. The appliance is typically air-tight except for the primary air and secondary air inlets into the primary and secondary combustion zones. Typically, a variable orifice/valve controls the flow of primary air to the various passage openings to the main firebox. The variable orifice/valve may be comprised of a generally tight sealing valve, for example, a sliding plate orifice system or rotating valve allowing various amounts of air to enter the main firebox. An electronic actuator is typically attached to this orifice, allowing for full-open or full-closed positions and any position in between full-open and full-closed. Therefore, the term “primary air” herein means the air stream provided to the main firebox at one or more locations in said firebox, mainly for the purpose of oxidation of the wood or other solid-fuel. In other words, the primary air is responsible for the primary combustion that occurs on or very near the solid fuel. It is responsible for the immediate elevation in temperature of the solid fuel in order to cause further outgassing. Under certain circumstances, excess oxygen from the primary air (not consumed by combustion in the main firebox) flows to the secondary combustion zone, described below, and may react in combustion of the volatiles reaching the secondary combustion zone. Preferred embodiments control primary air, which control has the synergistic effect of affecting the primary combustion of solid fuel (in the main firebox), the outgas sing of volatiles to the secondary combustion zone, and the amount of excess oxygen (in a “portion of the primary air stream”) leaving the primary combustion zone to arrive in the secondary combustion zone.

Preferred embodiments of the invention are adapted for use in appliances that, as a supplement to the main firebox, comprise a secondary combustion system/zone (also “secondary combustor” herein). Such secondary combustors are between the main firebox and the flue, that is, downstream (regarding gas flow) from the solid-fuel/wood chamber but upstream from the outlet of the appliance, which is typically a receiver ring that allows connection of the appliance to a flue pipe. The secondary combustor is typically above or behind the main firebox, in fluid communication with the main firebox. The secondary combustor is intended for supplemental, “secondary” combustion, specifically, burning of combustible gaseous compounds (“volatiles” from the fuel) inside the appliance but downstream from the main firebox. In other words, a secondary combustor in a wood-fired appliance is constructed with the specific purpose of introducing air, and/or working in conjunction with a catalytic element, in a proper amount to maintain its activation energy during the active combustion phase in order to combust the gas-rich mixture that is delivered from the firebox area. It specific location and construction are typically bound by maintaining this activation energy and delivering the necessary amount of air to maintain proper gas mixture that results in a near stoichiometric combustion that results in a clean combustion pattern.

It should be noted that, for proper operation of the secondary combustor, the temperature of the gases in the secondary combustor must be maintained at or above the activation energy of the specific type of combustor. For a tube-style combustor, this is generally approximately 1200 degrees F., and for a catalytic-type combustor, this temperature is above approximately 600 degrees F. The specific location of the combustor must take this activation energy into account therefore dictating it position in the stove. For example, this need for high temperature is one reason most secondary combustors are typically inside the cabinet and directly above the location wherein the solid-fuel/wood is burning.

The secondary combustor is typically constructed to receive additional air and/or to comprise catalytic element(s) to encourage combustion of the overly-rich mixture of gases that may be generated in the firebox, particularly during the organic combustion phase. For example, said additional air may be a secondary air stream, which is separate from the primary air and which is injected directly into the secondary combustor (see stream 27 in FIGS. 3 and 7, and/or simply “excess” primary air (or a “portion of the primary air”) that flows from the main firebox to the secondary combustion zone.

An important aspect of the preferred embodiments is the use of temperature measurement devices directly within the secondary combustion zone. The secondary combustion system may be selected, for example, from several variants commonly employed in the industry, for example, tube-style systems, catalytic element systems, and downdraft-style systems. It may be noted that an example of a tube-style system and an example of a catalytic element system are shown in the drawings. A downdraft-style system is not shown in the drawings, but will be understood by those of skill in the art; they are small boxes that are on the back of the appliance unit and basically force the volatiles thru a coal bed and then in a highly insulated box introduce secondary air to burn the gases.

Temperature measurement device(s) placed inside the secondary combustion zone provide temperature information to an electronic controller. The temperature information will typically comprise, consist essentially of, or consist of, the temperature of the gas that is directly contacting the measurement device, which typically may be considered representative of the temperature of substantially the entire secondary combustion area/zone. Alternatively, temperature measurement device(s) may be designed or placed so that it/they is/are in direct contact with, or covered/protected by, metallic or other heat-conducting structure inside the secondary combustion zone, rather than being in direct contact with the gasses; however, said metallic/heat-conducting structure and the covered/protected sensors are located generally centrally and appropriately in the zone so that the temperature sensed by said measurement device may be considered equal to the gas temperature, and, therefore, the temperature of the substantially the entire secondary combustion area/zone.

An important aspect of the preferred embodiments is that multiple phases, and preferably all three phases, of combustion of solid fuel are controlled. Upon reloading of fuel into the appliance, control of the start-up phase of combustion may comprise an operator depressing a “cycle start” button located on or near the appliance initiating a startup mode. A status indication light may be provided, for example, that illuminates after depression of the cycle start button to indicate that the unit is ready for a burn event. The startup mode of the controller preferably opens the primary air control all the way in order to provide ample air supply to the new load of fuel to ignite the fuel. Once the level of fuel combustion has progressed to a preset threshold point (also “high threshold” or “start-up threshold” SCT) measured by the temperature sensors in the secondary combustor, then the controller may in certain embodiments close the primary air orifice a certain amount in preparation for the modulation mode of the organic combustion. The preset high threshold SCT may be, for example, about 1200 degrees F. for a non-catalytic secondary combustion unit and for a catalytic combustion unit, regardless of the size of the fuel load, which represents a temperature wherein one may be certain that a good fire is going before further decisions are made.

Control during the organic phase of the combustion may be described as “modulation” control conducted in conjunction with operator input, preferably, the setting of a wall-mounted thermostat by the operator. The modulation control comprises the system monitoring and making decisions and consequential adjustments of the primary air orifice in order to adjust performance of the appliance, and repeating same until the SCT falls within a preset SCT target range, that is selected from two (a higher and a lower) preset SCT target ranges depending on whether the thermostat is calling for heat or not calling for heat, respectively. It may be noted that certain embodiments may be designed to respond to an operator's adjusting the thermostat in the middle of the burn cycle, for example, the operator changing the desired room temperature by changing the thermostat setting in the middle of the organic phase or even the charcoal phase. It will be understood from this document that the controller/algorithms (including code, programming, firmware, software, and/or hardware) would respond to the thermostat change as predetermined for that phase. For example, if the thermostat change were done in the organic phase, the controller would continue or begin iterative adjustments of primary air in response to the thermostat, so that the thermostat adjustment might result in the controller moving from a thermostat-satisfied SCT target range to a thermostat-not-satisfied SCT target range, or vice versa, at any point in the organic phase.

The cycle of monitor, decide and adjust includes an evaluation time period after adjustment, in order to give the performance of the appliance time to equilibrate. The evaluation time is established based upon the temperature of the secondary combustor at the time the modulation cycles are being conducted. If the SCT is much higher than the selected preset SCT target range, then a short evaluation time period (approximately 90 seconds, for example, a “short timer”) is used before making the ensuing decision to incrementally reduce (partially close) the primary air orifice. If the SCT is only slightly higher than the selected preset SCT target range, then a longer evaluation time frame (approximately 240 seconds, for example, a “long timer”) is used before making the ensuing decision to incrementally reduce (partially close) the primary air orifice. In either case, upon fulfillment of the selected timer (the end of the evaluation time), if the SCT is higher than the preset SCT target range, then the primary air orifice will be closed by a predetermined distance/amount via the electronic actuator. Having multiple timer options available allows the system to respond to a “typical” startup of generally dry wood and a cool firebox, or more quickly, for example, if the fire really “takes off” due to really dry wood or a hot reload. The iterative orifice closure distance is generally defined by the design of the primary air orifice, however generally will be from ½″ to 0.050″. If, at the time of fulfillment of the evaluation timer, the secondary combustion temperature is within the preset SCT target range, then no action is taken by the controller other than to restart the evaluation timers and continue the process. In normal operation/combustion, this multiple iteration, wait-evaluate-decide process will result, after some amount of time, in the primary air control having been closed to a preset minimum opening that is set within the controller. This minimum opening is preset in order to guarantee that sufficient primary air and oxygen will reach the secondary combustion zone to minimize emissions during the entire organic combustion phase. When this occurs, then, upon the fulfillment of the next evaluation timer event, no further closing will occur even if the secondary combustion temperature is above the preset range.

As mentioned above, input to the controller from the wall-mounted thermostat, of certain embodiments, determines what target SCT range is used during various portions of the organic phase modulation mode. While the thermostat is calling for heat, the controller uses a relatively high SCT target range that results in a relatively high heat output from the appliance. When the thermostat does not call for heat, indicating that the room has reached the desired temperature, then the controller switches to a relatively lower SCT target range that results in a relatively lower heat output from the appliance. In other words, during organic phase portions (periods) wherein the thermostat is satisfied, the controller closes the air flow iteratively according to the lower secondary combustion temperature range, that is, the range of a low burn. The iterative process (of measuring, adjusting, and then a timed evaluation (waiting) period before another measurement) is continued to reach whichever SCT target range is appropriate given the signal from the thermostat. It should be noted that fully-open or mainly-open primary air may be called-for while the thermostat is calling for heat, but at least partial closing, or full closure, of the primary air may be done even when the thermostat is calling for heat, if safety-based high-temperature limits that are held in memory of the controller are reached in the secondary combustion zone.

After the modulation mode of the system has satisfied the room temperature setting (thermostat setting) and has switched to a low burn by modulating to meet the lower SCT target range, the controller will typically make no further adjustments to the primary air control orifice while the organic combustion phase continues. The controller will typically make no further adjustments until the wood has burned to a great extent so that the SCT falls to another, significantly-lower preset temperature, that may be called the “charcoal-phase temperature” and that is preset as the indicator of the end of the organic phase of combustion and the start of the charcoal phase of combustion. The charcoal-phase temperature may be, for example, approximately 900 degrees F., as will be typical for the end of a successful burn during the organic phase. For example, for either a non-catalytic or a catalytic secondary combustion unit, the charcoal-phase temperature is expected to be about 932 degrees F. for a large load of fuel, or about 842 degrees F. for a small load of fuel.

Once the charcoal-phase temperature is reached, the preferred controller enters the charcoal phase of combustion control by closing the primary air orifice all the way, and then responds to true or false signals from the wall-mounted thermostat. If the thermostat is false (room temperature is above the set point) at any time during the charcoal phase of combustion, the controller will leave the primary air orifice closed all the way. If the thermostat becomes true (room temperature is below the set point), the controller will open the primary air control all the way in order to increase the charcoal burn to raise the temperature of the appliance, thus providing more heat to the room. Thus, it is the thermostat that signals the controller during most of the charcoal phase. After the initial closing based on SCT, the SCT is preferably ignored during this combustion phase. This is due to the fact that charcoal has virtually no organic compounds that would volatilize and enter the secondary combustor, so specific burn conditions related to obtaining clean combustion by optimizing the secondary combustor are negated.

Referring Specifically to the Drawings:

Referring to the drawings, there are shown several, but not the only, embodiments of the invented apparatus and/or methods of automatic control for a solid-fuel-burning, preferably wood-burning, appliance. The apparatus and/or methods are adapted to improve emissions, while achieving one or more of the goals of meeting a desired room temperature selected by an operator, increasing efficiency, and lengthening burn time of a single load of wood.

In general, “efficiency” herein is defined as the combination of combustion efficiency (how much chemical losses are realized in the flue stream, i.e. combustible gases that leave the appliance without being burned) and the heat transfer efficiency (which is how well the hot burned gases are cooled off while in the appliance prior to entering the flue). It may be noted that certain embodiments of the invented system are specially-adapted to affect efficiency mainly by maximizing combustion efficiency, specifically by minimizing chemical losses. Therefore, an efficient burn may be a fast burn or a slow burn, and certain embodiments of the invention may produce an efficient burn that is fast, or an efficient burn that is slow, for example.

One embodiment 1 of the invention is illustrated by the cordwood burning appliance in FIGS. 1 and 2, with an integrated system comprising a controller and actuator, temperature sensor(s), thermostat , and operator input switches (for example, a cycle start button) to provide control of heat output. Referring to the reference numbers in FIGS. 1 and 2, cordwood burning appliance 20 is constructed in a conventional fashion and employs a tube-style secondary combustion system 2. An embodiment of the invented combustion and emission control system is integrated into the appliance 20. It includes a combustion chamber 19 into which the cordwood fuel is loaded and combusted. An air tight door is not shown but is normal to the construction of a cordwood burning appliance 20. The secondary combustion system (secondary combustor) 2′ is shown in more detail in FIG. 3, and is a conventional style known in the art except for the adaptation of receiving temperature sensors 14, typically thermocouples, and/or protection for the sensors according to embodiments of the invention. A receiver ring 45 is attached to the appliance that allows for combustion by-products to exit the appliance into a flue or chimney (not shown in FIGS. 1 and 2). A controller 13 is responsible for the combustion control of the appliance and is located under and far away from the hot combustion chamber 19. Temperature sensors 14 located in direct proximity to the gases being combusted in the secondary combustor 2 send information to the controller 13 typically by means of a wired connection W1. After the controller interprets data from the temperature sensors 14, it sends a signal (via W2) to the electronic actuator 12 based on the controller/algorithm(s) (including “logic”, “programming”, “code”, “software”, “firmware” and/or “hardware”), discussed elsewhere in this document.

The status indication light 10 serves several functions. For example, if the status indication light 10 is illuminated after a fire has been recently started (in the start-up phase), it indicates to the operator that fuel can be added without having to press the cycle start momentary switch 11. Further, if light 10 is illuminated during the organic phase, it indicates that the SCT IS above the lowest target range of the controller (even if the target range being used by the controller at the time is a higher target range), and the operator may add more fuel to the stove without having to reset the cycle (without having to press the start button). This is because the inventor has found that if the SCT is above the lowest sct target range of the controller, there is enough thermal inertia to add more fuel to the stove and it will burn well even without having to reset the cycle. Thus, an operator may put “one more piece” on the fire before retiring to bed, for example, without restarting the cycle. The light 10 goes out once the SCT is below the lowest SCT target of the controller, indicating that if more wood is to be added the cycle should be reset. Also, the status indication light 10 serves the purpose of sending a blinking message to the operator that a fault has occurred, such as a burned out thermocouple, etc.

The cycle start momentary 11 pushbutton sends a signal to the controller 13 (via W3) that resets the control algorithm and opens the primary air orifice 16 to full open in anticipation of a re-fueling of the appliance. The electronic actuator 12 is attached to a control rod 15 that, by means of mechanical attachment to the primary air control rod 18, moves the orifice plates that are part of the primary air orifice 16. An operator selector unit 21, such as a heat output selection switch or a thermostat, is also connected to the controller 13 (via W4), and provides information to the controller 13 regarding the desired heat output or the room temperature, respectively.

In the organic combustion phase, the preferred thermostat (21) sends a signal to the controller 13 if the indicated (sensed) room temperature is lower than the thermostat set-point room temperature and the controller 13 controls the primary air orifice 16, according to the methods discussed elsewhere in this document, to attain a secondary combustor temperature (SCT) within a certain predetermined target range that is relatively high compared to a different, lower, predetermined SCT target range that is used if the thermostat is satisfied. This relatively high target SCT range typically requires the controller keep the orifice 16 entirely or substantially open, thus providing more air for combustion and raising the temperature of the room. Once the room temperature is high enough to satisfy the thermostat (21), a different signal sent to the controller 13 then controls the primary air orifice 16, again according to the methods discussed elsewhere in this document, to attain said lower predetermined SCT target range. This lower target SCT range typically allows/requires the controller to reduce (close an incremental amount) the orifice 16 an incremental mount once or iteratively until the lower SCT target range is obtained.

In the charcoal combustion phase, the controller again responds to the thermostat, but according to a somewhat simpler method, because the lack of organic volatiles escaping the charcoal to the secondary combustor allows for said simpler method. In this phase, if the thermostat calls for heat (room temperature lower than the set-point), then the controller opens the orifice. If the thermostat is satisfied (room temperature at or higher than the set-point), then the controller closes the orifice, typically all the way to 100% closed to conserve the remaining charcoal fuel.

FIG. 3 shows a close up view of the tube style secondary combustion system 2 of FIGS. 1 and 2, removed from the appliance 20. Metered or unmetered secondary air 27 (in dashed line) enters the manifold 24 that is generally sealed to the side of the combustion chamber 19 (see FIG. 2). The secondary air is then distributed to the individual tubes 22 and then enters the secondary combustion zone (FIG. 2). The baffle board 23, a generally horizontal board above the tubes 22, serves to block the gasses from rising and diverts the gases to flow generally horizontally along/across multiple tubes 22 toward the front of the appliance (FIG. 2). Temperature sensors 14 are located in a separate tube 26, typically parallel to the tubes 22, and between two of the tubes, that serves to hold and protect the sensors 14. Secondary combustion of the combustible gasses may take place at or near the tubes 22 and board 23, given appropriate oxygen content and temperature in that zone 2. The temperature sensors 14 monitor the temperature of the mixture of gasses (air and combustibles or “volatiles”) and this temperature is the “secondary combustion temperature” (SCT) discussed herein. In this embodiment, two sensors 14 are shown, but other numbers of sensors could be used. Other means to hold and protect the temperature sensors may be used.

Another embodiment 1′ of the invention is illustrated by the cordwood burning appliance in FIGS. 4 and 5, wherein the combination of cordwood appliance 20′ and the combustion and emission control system includes much the same equipment as embodiment 1 in FIGS. 1 and 2 (hence, many reference numbers are the same), except that an alternative embodiment of the secondary combustor is used, specifically, a catalytic combustion system 2′. Referring to the reference numbers in FIGS. 4 and 5, cordwood burning appliance 20′ is constructed in a conventional fashion and employs a catalytic secondary combustion system 2′ such as is known in the art. An embodiment of the invented combustion and emission control system is integrated into the appliance 20′. The appliance includes a combustion chamber 19 (also “main firebox”) into which the cordwood fuel is loaded and combusted. An air tight door is not shown but is normal to the construction of a cordwood burning appliance 20′. The secondary combustion system (secondary combustor) 2′ is shown in more detail in FIG. 6, and is a conventional style known in the art except for the adaptation of receiving temperature sensors 14, typically thermocouples, and/or protection for the sensors according to embodiments of the invention. A receiver ring 45 is attached to the appliance that allows for combustion by-products to exit the appliance into a flue/chimney F, shown in dashed lines in FIG. 5. A controller 13 is responsible for the combustion control of the appliance and is located under and far away from the hot combustion chamber 19. Temperature sensors 14 located in direct proximity to the gases being combusted in the secondary combustor 2′ send information to the controller 13. After the controller interprets data from the temperature sensors 14, it sends a signal to the electronic actuator 12 based on the controller algorithm(s), discussed elsewhere in this document.

FIG. 6 shows a close up view of the catalytic element 30 of system 2′ of FIGS. 5 and 6. The catalytic element 30 is generally located in the same location as the tubes 22 and baffle board 23 of the secondary combustor 2 of FIGS. 1 and 2. Directed to flow generally horizontally forward toward the front of the appliance 20 by the horizontal baffle board 23′ (see FIG. 6), air 32 and combustible gases 33 flow from the combustion chamber 19 to the front of, and then rearward through, the catalytically-active portion 130 of element 30. Top flange 131 and side flanges 133, protruding forward from the catalytically-active portion 130, further serve to direct the air/gas flow to portion 130. The temperature of the mixture of gasses (air and combustibles or “volatiles”) is monitored by temperature sensors 14′, typically thermocouples, and this temperature is the “secondary combustion temperature” (SCT) discussed herein. The temperature sensors 14′ can be located in the front of the catalytic element 30 (as shown in FIG. 6). Or, temperature sensors 14′ may also be located in the space behind the catalytic element 30 but still upstream of the ring 45 and flue F, or a combination thereof (as shown schematically in dashed lines in FIG. 5). The catalytic element 30 serves as a location of oxidation of the combustible gases 33 and air 32. The by-product gases 34 then exit the catalytic element 30 and exit the appliance 20′ via the receiver ring 45 that is attached to a flue or chimney F.

FIG. 7 shows the air flow and combustible gases that are typical of the preferred appliances in which many embodiments of the invention will be built/retrofit. The primary air streams 40, 41 and 42, all being air that is provided through primary air orifice 16, is distributed to multiple locations within the combustion chamber 19 (main firebox) in various proportions. For example, primary air streams 40 and 41 are directed to enter the chamber 19 generally below the burning solid-fuel/wood 46. Primary air stream 42 is split-off from the other primary air, and directed inside conduit/passageways inside the cabinet of the appliance, to flow through an inlet 142 at a front side of the appliance but still supplying the chamber 19 with oxygen for solid-fuel/wood 46 combustion. Therefore, all three streams 40, 41, and 42 are controlled by the primary air orifice 16 and all three streams 40, 41 and 42 are brought in contact with the solid-fuel/wood 46. Direct combustion occurs on the solid-fuel/wood 46, as well as outgassing of combustible gases 47 that are carried upwards by buoyancy to the secondary combustion system 2. Secondary air 27 (see FIG. 3) is introduced into the secondary combustion system (in this example, through tubes 22 as in FIG. 3), that is, above the solid-fuel/wood 46 and in a position to mix easily with the combustible gases 47 rising up from the fuel 46. Thus, burning of the combustible gases 47 occur in the region of the secondary combustion system 2 (FIG. 1). The temperature of the combustion process is monitored by temperature sensors 14 (see FIG. 1). The by-product gases 44 then travel in a cavity formed between the top 48 of the appliance and the baffle board 23 (see also FIG. 3). The top 48 of the appliance serves as the main heat exchanger in order to provide heat to the ambient environment/room. The by-product gases 44 then exit the appliance through the receiver ring 45. Thus, the gasses 44 may be said to exit the appliance before reaching the flue/chimney, and it is clear that the secondary combustion system and zone, and the associated temperature sensors, are located inside the appliance, upstream of the appliance's exit to the flue. Preferably, the temperature sensors are located within a few inches of the secondary combustion zone structure, for example, within 0.5-4 inches, or within 1-3 inches of a secondary air tube 22, baffle board 23, or catalytically-active portion 130. In many embodiments, the controller does not receive any messages from any temperature sensors (is any exist) in the primary combustion zone.

It is clear that the preferred temperature sensors for sensing secondary combustion are not in the flue. It is clear too that the preferred temperature sensors for sensing secondary combustion are neither near nor contacting, nor sensing the temperature of, the appliance cabinet, which may also be called the appliance “cabinet walls” or “outer housing”.

FIG. 8 shows examples of some, but not all, heat-output scenarios, wherein the different scenarios are created by the controller using different predetermined SCT targets. Each SCT target is typically a range of SCT temperature, for example, a range of about 120 degrees F. For example, for a non-catalytic secondary combustion zone, a relatively high heat output mode H could use a high SCT target range of 1500 degrees F. to 1380 degrees F., a relatively medium heat output mode M could use a high SCT target range of 1370 to 1250 degrees F., and the relatively low heat output mode L could use a low SCT target of 1180 to 1060 degrees F. For example, for a catalytic secondary combustion zone, a relatively high heat output mode H could use a high SCT target range of 1470 degrees F. to 1350 degrees F., a relatively medium heat output mode M could use a high SCT target range of 1120 to 1000 degrees F., and the relatively low heat output mode L could use a low SCT target of 770 to 650 degrees F. It may be noted that the differences between each range, that is, from the lowest temperature of one range to the highest temperature of the next-lowest range, is in the range of 150-250 degrees F., or in the range or 200-250 degrees F., or preferably about 230 degrees F. It may be noted that these high SCT target ranges are higher than the preset high threshold temperature of about 1200 degrees F., discussed above. This is because the fire continues to build in temperature after the first small closing of the primary air valve (at the end of shutdown), so that the SCT will continue to build for a time after said first small closing, so that, even with said first small closing, the fire will take the SCT Into the high SCT target range if the a high heat output selection or thermostat setting is commanding the high SCT target range.

The high and low modes H, L, may be used to illustrate the high and low SCT target ranges used in preferred embodiment, with selection of the high SCT target range or the low SCT target range being based on whether the thermostat is not satisfied and therefore calling for more heat output, or whether the thermostat is satisfied because the desired room temperature has been met.

Therefore, FIG. 8 illustrates the primary orifice relative to time for the various target heat output scenarios. In high heat output mode H, the orifice stays wide open (start-up phase, P1) until a predetermined/preset (high) threshold temperature is realized in the secondary combustor, as measured by the secondary combustion zone temperature sensors. The orifice is then closed a small amount (at C1) and the “high burn” SCT target referenced in the control algorithm is used to control any further closing of the primary air orifice during the modulation of the organic combustion phase P2-H. This high heat output mode H in FIG. 8 would generally correspond to a scenario wherein the thermostat continues to call for heat throughout most of the burn so that a high target SCT range is selected and maintained. At the end of the organic burn phase P2-H, another (low) temperature threshold is reached (see temperature 65 in FIG. 9), indicating that the fuel is gone and the orifice can then be shut all the way to closed (at C2) assuming the thermostat is satisfied by the end of the organic phase P2-H.

The same/similar operation occurs for the low heat output mode L illustrated in FIG. 8, which except that a lower preset SCT target range is referenced in the control algorithm, for example, because the thermostat indicates the desired room temperature has been met. In this low heat output mode L, the orifice stays wide open (start-up phase, P1) until a predetermined/preset (high) threshold temperature is realized in the secondary combustor, as measured by the secondary combustion zone temperature sensors. The orifice is then closed a small amount (C1) and, assuming the thermostat is satisfied in this scenario, the “low burn” SCT target range is referenced in the control algorithm is used to control any further closing of the primary air orifice during the modulation of the organic combustion phase P2-L. In this low heat output mode L, the controller commands the electronic actuator to close further than it does in the high heat output mode H, due to the lower SCT target range that is referenced. This low heat output mode L would generally correspond to a scenario wherein the thermostat is set so low, or the room is already so warm, that the thermostat immediately/soon does not, and continues not to, call for heat throughout most of the burn, resulting in the low target SCT range being selected and maintained. At the end of the organic burn phase P2-L, another (low) temperature threshold (for example, 65 in FIG. 9) is reached indicating that the fuel is gone and the orifice can then be shut all the way to closed (starting at C2 in FIG. 9). Assuming the thermostat is still satisfied through the charcoal phase P3-L, the orifice will remain closed.

Therefore, the steep change in slope after C2, in FIG. 8, represents the beginning of the charcoal phase of combustion (P3-H and P3-L) due to no further combustible gas compound from the solid-fuel/wood (“volatiles”) being available for the secondary combustor to burn. Near the end of the organic combustion phase P3-L of the low heat output mode L, for example, when the SCT first drops to the low threshold temperature, the controller opens the primary air orifice very slightly (at O-L) in order to raise the temperature of the fuel load slightly and allow for further outgassing of volatiles. Soon after that, the low temperature threshold is reached again in the low heat output mode L, and the primary air orifice is closed all the way.

One may note in FIG. 8 that the low heat output mode exhibits a shorter organic phase than the medium and high heat output modes, that is, more quickly reaching the point wherein the primary air valve is reduced to zero percent open. This may occur because, in the low mode, the stove is essentially “baking” the wood, that is, by-and-large acting like a gasifier. The gross result is that, fairly quickly, the volatiles are baked out of the wood and a larger portion (compared to medium and high modes) of charcoal is left that has no smoke (no volatiles). Thus, more quickly than in medium and high mode, the low threshold temperature is reached and the controller can shut down the primary air (if the thermostat is satisfied in embodiments with a thermostat). On the other hand, during high or medium mode, the organic phase is a (longer) combination of burning through a significant portion of the charcoal plus and burning volatiles, and this combination of burning charcoal while burning volatiles typically makes it necessary to hold the primary air orifice at least partly open for a longer time. The fact that the primary air control goes to zero at an earlier time in the low heat output mode does not mean that heat output has stopped, it means that heat output is slowed down, but that charcoal BTU's will be subsequently produced over a longer charcoal phase (typically, a longer charcoal phase than in the medium and high modes).

FIG. 9 is a graph that depicts the relationship of the temperature (SCT) of the gases in the secondary combustor in relation to time for a low heat output setting, for example, such as low heat output mode L in FIG. 8. The steep line 60 shows the rise in temperature of the gases after a refueling event has occurred; this region is defined as the startup phase of combustion P1-L. The temperature builds until it reaches the upper threshold temperature depicted by 61. The upper threshold temperature 61, as shown in FIG. 9, may be at the upper limit of the SCT target range. Alternatively, in certain embodiments, the upper threshold temperature 61′ may be within or lower than the preset SCT temperature range (TR-L), due to the fact that the fire may continue to increase SCT even after a first small closing of the primary valve, as discussed above. Or, alternatively, in certain embodiments, the upper threshold temperature 61″ may be higher than the preset SCT temperature range (TR-L). As explained elsewhere in this document, an example upper threshold temperature 61 may be 1200 degrees F.

This following portion of combustion is referred to as the organic combustion phase P2-L, or “organic portion”. This first peak then dips (trough 62) due to the controller closing primary air control valve some set amount (see C1 in FIG. 8), which in this scenario brings the SCT into the SCT target range TR-L. The temperature then builds again (peak 63) and the controller closes the primary air valve again to drop (trough 64) the SCT to the predetermined level, that is, to be substantially within, and then preferably entirely within, the SCT target range TR-L. As will be understood by viewing FIG. 9, the iterative process of bringing SCT to the SCT target range may comprise SCT being only slightly outside (up to about 50 degrees F.) the SCT target range for short periods of time, for example for up to about 5 minutes for each occurrence, and this is within the definition of the term “substantially within” in this context; such iterations would typically use the “long timer”. In other situations, iterations may comprise SCT being greatly outside (for example, 100-300 degrees) the SCT target range for a few iterations, in which case the “short timer” would be used to make quicker adjustments in order to bring, after said few iterations, the SCT to be substantially within, or entirely within, the SCT target range. The iterations continue until the primary air orifice is either closed to a programmed (preset) minimum orifice opening or until the SCT remains within the TR-L without further adjustment (see the flat slope in FIG. 8). It may be noted from FIG. 9 that more than 4 iterations of closing, evaluating, and readjusting the primary air valve are done, but other numbers of iterations may be done in certain embodiments and depending on the size and type of solid fuel load, for example. For example, one might expect 1-8 iterations, or 2-7 iterations, or 3-6 iterations, in certain embodiments for each change in the SCT target range. For a scenario wherein a large load of fuel is placed in an appliance in a room with a thermostat and easily or moderately-easily heated by the appliance, one might expect two SCT target ranges to be used, that is, the high range in the beginning of the organic phase and the low range once the thermostat is satisfied. In such a scenario, for example, one might expect a total of 4-14 (twice times 2-7) iterations for the two ranges combined, or more typically 6-12 (twice times 3-6) iterations, between the two ranges.

The air control is then held stationary (see flat slope in FIG. 8) until the end of the organic phase, indicated by the steep downward temperature slope 66 and reaching of the low threshold temperature 65, due to the fuel being exhausted and the temperature of the secondary combustor lowering naturally. The low threshold temperature could be, for example, about 932 degrees F. for a large fuel load initially placed into the appliance, or about 842 degrees F. for a small fuel load initially placed into the appliance, regardless of whether the non-catalytic or catalytic secondary combustion zone is being used. This begins the stage of combustion referred to as the charcoal phase P3-L. The primary air control is then closed all the way via the controller. In certain embodiments, as discussed earlier, the controller may open the primary air during the charcoal phase P3-L, if the thermostat calls for heat.

In certain embodiments, there may be situations in which, after the first, small closing of the primary air, the SCT drops quickly in the organic phase to a level that is below the lowest SCT target range; this may occur when a very small amount of fuel is placed in the appliance for the first and only fuel load, for example. In such cases, the controller will not further adjust the primary air, until a value of SCT is reached that triggers entrance into the charcoal phase. Due to the single adjustment of the primary air (said first, small closing), the controller may select a lower-than-normal “value of SCT” to signal the start of the charcoal phase. In other words, the controller may select a lower than the normal low threshold temperature (charcoal phase temperature) for such cases wherein there are very few iterative adjustments of the primary air because the SCT drops quickly. In such situations, the fire is still clean, but the secondary combustor is not engaged (or minimally) and the fire is burning more like a fireplace rather than an airtight wood stove.

FIGS. 10 and 11 illustrate a preferred embodiment, which provides a scenario wherein the appliance is controlled for high heat output, until the desired room temperature is met according to the user's thermostat, and then the appliance is controlled for lower heat output. FIG. 10 illustrates the primary orifice relative to time for one example of this preferred scenario, wherein a high heat output mode H is used during the beginning of the organic combustion phase, but said high heat output heats the room to the point where the thermostat is satisfied midway through the organic combustion phase. When the thermostat is satisfied, the controller switches at transition T to referencing the low heat output SCT target. Then, the controller iteratively closes the primary air orifice until the SCT falls within the low target range, and the organic phase continues in a low heat output mode L such as described above for FIG. 8. FIG. 11 illustrates the SCT profile during the primary air orifice adjustments of FIG. 10. This embodiment allows for convenient heating of the room to a desired temperature, but, after that, maintenance of that room temperature typically may be done by controlling the appliance to output less heat while still reducing/minimizing emissions. Thus, this scenario comprises a start-up phase P1 and a first portion of organic phase P2 typical of a high heat output mode H, but a latter portion of the organic phase P2 and an organic phase P3 typical of low heat output mode L.

FIG. 12A-C illustrate methods of control and operation of certain embodiments, which methods have been generally or specifically discussed above. One may note that FIGS. 12A-C use the term “selected SCT target range TR”, wherein selection is a broad term that may include several types of “selection”.

The preferred embodiments utilize, in FIGS. 12A-C and 13, a thermostat in a room that the user will use in a conventional manner to enter a set-point corresponding to the desired room temperature. The true or false signals from the wall-mounted thermostat, sent to the controller, determine whether what SCT target range TR is “selected” by the controller/algorithm(s). As illustrated by FIG. 13, if the thermostat changes its signal partway through the organic combustion phase (with or without the user changing the thermostat), this results in the new “selection” of SCT target range, and a new iterative process for bringing the SCT into compliance with the newly-selected SCT target range.

In alternative embodiments, the selection, in FIGS. 12A-C and 13, could be done by other methods/apparatus. For example, certain embodiments may be adapted so that the user selects a heat output via a switch, for example, a wall switch or switch near the controller. The controller then responds by using a SCT target range preprogrammed for that heat output. As illustrated by FIG. 13, if the user changes the heat output selection changes partway through the organic combustion phase, this results in the new “selection” of SCT target range, and a new iterative process for bringing the SCT into compliance with the newly-selected SCT target range.

This may result in an appliance that can operate in all of the high, medium, and low modes of FIG. 8, and may result in an operation wherein the operator may switch the heat output selection in the middle of the burn cycle, for example, in the middle of the organic phase. One may understand from this document, including the disclosure of the provisional application, that said switching would result in iterative adjustments of primary air to reach the newly-selected heat output mode. Examples of such adaptations and methods are focused on in the provisional application of which this non-provisional claims priority, that is, Provisional Application Ser. No. 61/851,343, filed Mar. 7, 2013, incorporated herein by reference.

The main objective of the preferred embodiments is to provide ease of use to the user and ensure clean emissions at all available burn rates/heat-output modes. The electronic-based system may use a parameter table, for the high threshold temperature that triggers/signals the end of the start-up phase, the multiple SCT target ranges, the low threshold temperature that triggers/signals the end of the organic phase, and failsafe temperature safety limit(s) to ensure that the automatic operation of the appliance does not enter dangerous scenarios. The parameters/table may be established by a manufacturer and/or retrofitter, and the controller may be programmed without undue experimentation, given the teachings of this document and the figures. For example, fine-tuning of the parameters, relative to those disclosed herein, may be determined for each appliance model either without undue experimentation, and then the parameters would be entered into memory of the controller during the manufacturing process of the controller.

The process of evaluating the temperature of the gases in the secondary combustor via a wait, evaluate and make decision process is not the only method of control possible when considering the present invention. Other algorithms based on PID (proportional-integral-derivative) control of the relationship between secondary gas temperature and control of the primary air orifice may be used in certain embodiments. The process of adjusting primary air into the combustion chamber of a cordwood fired appliance based upon gas temperature of the secondary combustor is important to most or all embodiments of the invention, and may be incorporated into several types of secondary combustors and ones not yet invented. If the gas temperature of the secondary combustor is sufficient then by proxy the appliance is burning clean and efficiently. Secondarily, this gas temperature also indicates various stages of the combustion process and decisions can be made by the controller to influence burn time and heat output.

It will be noticed that the preferred embodiments do not comprise adjustment or other control of secondary air. Thus, certain embodiments may be said to be control apparatus and/or methods that comprise, consist essentially of, or consist of adjusting primary air in response to one or more temperatures measured inside or immediately adjacent to the secondary combustion zone of a solid-fuel burning appliance. Said temperature will be the temperature, or extremely close to the temperature, of the gasses burning in the secondary combustion zone, which will be a mixture of gasses from the main firebox burning of solid fuel, including “excess” primary oxygen that is not consumed in the main firebox and combustible volatiles from the wood, and in some, particularly unanalyzed embodiments, secondary air added directly to the secondary combustion zone. Thus, it is important to note that the preferred embodiments control air stream(s) added to the appliance for combustion of the solid fuel in the main firebox, but that this primary air control has a synergistic effect of controlling and improving secondary combustion zone performance and emissions performance of the appliance.

In certain embodiments that comprise adjusting primary air according to teachings herein, secondary air stream(s) also may be controlled via the same control system or a supplemental control system. Therefore, in certain embodiments of the invention, it may be advantageous to also control the air stream(s) that enter(s) directly into the secondary combustor (in addition to controlling the primary air that first enters the main firebox for solid-fuel combustion) in order to maximize efficiency and to control SCT even further. The inventor believes that if a system were built to control only secondary air, rather than primary air and possibly also secondary air, then the unit would be “stuck on high” due to there being no control of the primary combustion zone (main firebox) and the level of volatile gas production.

Although this invention has been described above with reference to particular means, materials and embodiments, it is to be understood that the invention is not limited to these disclosed particulars, but extends instead to all equivalents within the scope of the following claims. 

1. A control system in a solid-fuel-burning appliance for reducing emissions, the appliance having a primary combustion zone for receiving and burning solid fuel, a secondary combustion zone for burning combustible compounds outgassed from said burning of solid fuel, a primary air stream entering the primary combustion zone wherein a portion of the primary air stream flows with the outgassed organic compounds to the secondary combustion zone, and a primary air valve openable to increase flowrate of the primary air stream and closable to reduce flowrate of the primary air steam, the control system comprising: a temperature sensor located in the secondary combustion zone and measuring secondary combustion zone temperature (SCT); and a controller adapted to receive a signal from said temperature sensor indicating measured SCT, wherein the controller is adapted to adjust flowrate of the primary air stream in response to the signal to raise or lower said SCT.
 2. A control system as in claim 1, wherein the controller is adapted to adjust said flowrate in incremental amounts in response to said signal until the measured SCT is within a predetermined SCT target range.
 3. A control system as in claim 2, wherein the SCT target range is a range of about 120 degrees Fahrenheit.
 4. A control system as in claim 1, wherein said controller is adapted to adjust said flowrate in incremental amounts in response to said signal until the SCT is within a first range selected from a plurality of predetermined SCT target ranges.
 5. A control system as in claim 4, wherein said plurality of predetermined SCT target ranges comprises at least two SCT target ranges selected from a group consisting of: a high SCT target range for high heat output from the appliance, a low SCT target range for low heat output from the appliance, and a medium SCT target range, between said high and low SCT target ranges, for medium heat output between said high and low heat outputs, and wherein said first range is selected from said plurality of SCT target ranges by means of at least one signal from an operator-controlled unit selected from the group consisting of a heat-out selection switch and a thermostat.
 6. A control system as in claim 1, comprising a room thermostat and wherein said controller is adapted to adjust said flowrate of the primary air stream in response to the signal from said temperature sensor until the measured SCT is within a higher SCT target range when the thermostat is calling for heat and until the measured SCT is within a lower SCT target range when the thermostat is not calling for heat.
 7. A control system as in claim 6, wherein, when the thermostat is not calling for heat, said controller incrementally adjusts said flowrate of primary air until the first of: the SCT falling within the lower SCT target range or the valve closing to a preset minimum valve opening.
 8. A control system as in claim 6, wherein the controller includes a preset low threshold temperature, which generally corresponds to SCT at an end of an organic release phase of solid-fuel combustion in the primary combustion zone and the beginning of a charcoal combustion phase in the primary combustion zone, and wherein, when said signal from the temperature sensor indicates that the measured SCT has fallen to said preset low threshold temperature, the controller is adapted to completely close the valve to stop the primary air stream from flowing into the primary combustion zone.
 9. A control system as in claim 8, wherein the controller is adapted to increase the primary air stream flowrate, after the end of the organic release phase, only if the thermostat calls for heat.
 10. A control system as in claim 1, wherein the primary air valve is a sliding plate system forming one or more orifice openings.
 11. A method of controlling emissions from burning of wood, the method comprising: providing a solid-fuel-burning appliance having a primary combustion zone receiving and burning solid fuel, a secondary combustion zone for burning combustible compounds outgassed from said burning of solid fuel, a primary combustion air stream entering the primary combustion zone wherein a portion of the primary combustion air stream flows with the outgassed organic compounds to the secondary combustion zone, and a primary air valve openable to increase flowrate of the primary air stream and closable to reduce flowrate of the primary air steam, and a secondary air stream supplied into the secondary combustion zone; providing a temperature sensor located in the secondary combustion zone and measuring second combustion zone temperature (SCT); and adjusting flowrate of the primary air stream in response to measured SCT to maintain the secondary combustion zone above a temperature required for combustion of volatiles from the primary combustion zone.
 12. A method as in claim 11, wherein the method does not comprise controlling flow of the secondary air stream.
 13. A method as in claim 11, wherein said adjusting comprises iteratively adjusting said flowrate in incremental amounts until measured SCT is within a predetermined SCT target range.
 14. A method as in claim 13, wherein the SCT target range is a range of about 120 degrees Fahrenheit.
 15. A method as in claim 13, wherein said adjusting comprises adjusting said flowrate in incremental amounts until measured SCT is within a first range selected from a plurality of predetermined SCT target ranges.
 16. A method as in claim 15, wherein said plurality of predetermined SCT target ranges comprises at least two SCT target ranges selected from a group consisting of: a high SCT target range for high heat output from the appliance, a low SCT target range for low heat output from the appliance, and a medium SCT target range, between said high and low SCT target ranges, for medium heat output between said high and low heat outputs, and wherein said first range is selected from said plurality of SCT target ranges by means of a signal from an operator-controlled unit selected from the group consisting of: a heat-out selection switch and a thermostat.
 17. A method as in claim 15, comprising providing a room thermostat and said adjusting flowrate of the primary air stream is done until measured SCT is within a higher SCT target range when the thermostat is calling for heat and until measured SCT is within a lower SCT target range when the thermostat is not calling for heat.
 18. A method as in claim 17, comprising, when the thermostat is not calling for heat, adjusting said flowrate until the first of the measured SCT falling within the lower SCT target range or the valve closing to a preset minimum valve opening.
 19. A method as in claim 17, comprising completely closing the valve to stop the primary air stream from flowing into the primary combustion zone when measured SCT falls to a preset low threshold temperature that generally corresponds to SCT at an end of an organic release phase of solid-fuel combustion in the primary combustion zone and the beginning of a charcoal combustion phase in the primary combustion zone.
 20. A method as in claim 19, comprising increasing the primary air stream flowrate, after the end of the organic release phase, only if the thermostat calls for heat.
 21. A method of limiting emissions from a wood-burning appliance having a primary combustion zone for burning wood, and a secondary combustion zone for burning volatiles outgassed from the burning wood, the method comprising: measuring secondary combustion zone temperature during a wood-burning event, and adjusting primary air supplied to the primary combustion zone to maintain the measured secondary combustion zone temperature above a temperature corresponding to sufficient activation energy for combustion of the volatiles.
 22. A method of claim 21 wherein the secondary combustion zone is provided with secondary air flow, and wherein the secondary air flow is not adjusted in response to the measured secondary combustion zone temperature.
 23. A method of claim 21, wherein the wood-burning event comprises a start-up phase, an organic release phase, and a charcoal-burning phase, and wherein said adjusting of primary air occurs during the organic release phase to combust said volatiles, and occurs during the charcoal-burning phase only in response to a room thermostat calling for heat. 